Conductive materials

ABSTRACT

The invention relates to conductive heaters, and has its objective to provide a material that exhibits a uniform heating effect over the full area of the heater, relatively free from hot and cold spots, a further objective being to maintain the heater pliable. These objectives are met by a conductive material comprising finely divided carbon particles uniformly dispersed in an elastomeric carrier there being carbon particle levels of 20% to 75% by dry weight to carrier levels. Preferably, the carrier is an elastomeric polymer. A further aspect of the invention is a method of forming a compound for an electrically conductive heater comprising stirring fine carbon particles into a polymer base containing an anti-adsorption compound, to achieve carbon particles to polymer levels of 20% by dry weight to 75% by dry weight, and subjecting mixture to high speed stirring for a pre-determined period of time, with the maintenance of the mixture below a predetermined level, to grind the carbon particles to a predetermined final fineness.

This invention relates to conductive materials and is particularlyconcerned with conductive materials to serve the purpose of a heatingmeans.

It has long been known that materials that are electrically conductivecan generate heat. This phenomenon has spawned a considerable number ofheating means for a considerable number of different purposes.

Whilst electrical heaters at large are generally successful, and morethan capable of meeting their intended purpose, what has proved to bedifficult, is the achieving of a uniform heating effect over arelatively wide area, substantially free from hot and cold spots orareas.

There have been attempts hitherto to create electrical heaters in sheetor web form to serve a wide variety of purposes. However, the mostusable form of conductive material is in the form of carbon particles orcarbon black embedded in or coated on a carrier or substrate. To attemptto achieve a uniform heating effect, a greater density or concentrationof carbon particles is employed, but as a direct result of that, thematerial becomes less pliable and more brittle, to the detriment of theemployment of the material over a wide range of potential uses.

The first object of the present invention is to provide a start or batchmaterial able to be used in a variety of different physical forms tosuit a wide variety of electrical conduction heaters.

Second and subsequent objects of the invention are to transform thestart or batch material into products that are electrical conductiveheaters for a wide variety of different purposes.

According to a first aspect of the present invention, a conductivematerial comprises finely divided carbon particles uniformly dispersedin an elastomeric carrier there being carbon particle levels of 20% byweight to 75% by dry weight to 80% to 25% by dry weight of carrierlevels.

Carbon particle levels of 20% to 40% by dry weight to 80% to 60% by dryweight of carrier levels can be used.

The carbon particles may be so-called carbon black, a finely dividedpowder, and the elastomeric carrier is an elastomeric polymer, andaccording to a further feature of the invention, a semi-conductive filmor coating comprises a carbon filled elastomeric polymer, preferablywith carbon particle levels of 43% to 73% by dry weight to 57% to 27% bydry weight polymer levels, preferably 60% by dry weight to 65% by dryweight. Further preferably, the carbon particle level is 57% by dryweight.

The elastomeric polymer may be an aliphatic polyurethane in solution,and desirably the carbon particles are mixed without milling prior tothe addition thereto of the polyurethane solution.

If required in particular applications, flame retardant materials may beprovided.

According to a further aspect of the invention a method of forming acompound for an electrically conductive heater comprises stirring finecarbon particles into a polymer base containing an anti-adsorptioncompound, to achieve carbon particles to polymer levels of 20% by dryweight to 75% by dry weight to 80% to 25% by dry weight of polymer, andsubjecting mixture to high speed stirring for a pre-determined period oftime, with the maintenance of the mixture below a predetermined level,to grind the carbon particles to a predetermined final fineness. Thepredetermined level may be at not more than 25° C.

The required carbon particle level may be 20% to 40% by dry weight topolymer level but preferably carbon levels are from 43% to 73% by dryweight to polymer, and more particularly 57% by dry weight.

The resultant mixture exhibits thixotropic rheological properties, andif required, the viscosity of the mixture can be reduced, by theaddition of a suitable solvent such as dimethylformamide.

Preferably the carbon black particles have particulate size ofapproximately 30 En nm. Suitable adsorbants may be selected from thegroup containing polypropylene glycols, and polyethylene glycols of arequired molecular weight. After this process is complete a suitablepolymer solution eg an aliphatic polyurethane is added to the masterbatch such that the ratio of polymer to carbon black is 1:0.57 on a drybasis. The finished compound is then refiltered prior to any coatingprocess.

The duration of the slow stirring-in of carbon black is not critical,but desirably high speed stirring is limited to not more than 30minutes.

Once the mixture is prepared, it is checked by taking a sample andpassing it through a 200 mesh filter, preferably with the aid of a lowpressure displacement pump, and checked by using a Heckman gauge, toensure that there has been no agglomeration of the carbon particlesduring mixing. If any agglomeration is detected, the mixture should besubjected to further high speed stirring.

Preferably, the prepared start or batch material is subjected to a finalfiltration step by passing the mixture through a 300 mesh filter cloth,for example by way of a slow, low pressure positive displacement pump,at which stage there should be no residue left on the filter cloth thatwould signal that there was still a degree of remnant agglomeration ofcarbon particles.

In the form where the start or batch material employs dimethylformamideas the solvent and polyethylene or propylene glycol as the polymer base,it constitutes an ideal material to serve as a coating or a base orcarrier material.

According to a further aspect of the invention a web or sheet is formedby applying a quantity of finished compound as discussed above to arelease paper by way of transfer coating, to achieve a uniform coatingor film of compound between 90 and 100 grams per square meter dryweight, and subjecting the web or sheer to heat progressively risingfrom 110° C. to 150° C. to achieve the controlled release of solventsand provide a coating or film free of pinholes.

As the electrical conductivity, and hence the heating effect achievableis a function of coating or film thickness the above process is repeateduntil a desired thickness of coating or film is created.

At this stage considerable care must be exercised to ensure thatreticulation is avoided, and as it is preferred to spread the finishedcompound on the release paper by employing a doctor blade, equallyconsiderable care exercised to ensure the avoidance of the presence ofdirt or grit on the blade edge, to prevent the creation and spread oflines of indentations in the coating or film.

Desirably, the release paper is matt grade and is an unembossedsilicone-coated paper.

According to a still further aspect of the invention, a flexible fabricable to serve the purpose of an electrical conductive heater is formedby taking the release paper mentioned above with its coating of finishedcompound spreading thereon a further quantity of said compound, layingthe release paper on a flexible fabric carrier sheet or web, and passingthe composite through a fixed gap roller to ensure controlledpenetration of said compound into the fabric of the sheet or web, thesheet or web thereafter being subjected to heat progressively risingfrom 110° C. to 150° C. to achieve controlled release of solvents andprovide a coating of film free of pinholes. Also possible is the directapplication of a coating of finished compound directly on to a fabriccarrier.

Such a sheet or web can be of any desired length, and of any width withthe limits of available fixed gap rolls.

By the nature of the coat or film, there is a substantially totally evenspread of carbon black at loadings in the polymer material considerablybeyond that which has hitherto been believed to be possible whilst atthe same time leaving the fabric with its film or coat totally flexible.

The fabric may be a knitted cotton material but can be of any othersuitable form, such as a weft knitted polyvinyl alcohol fabric.

The preparation of the said compound and the manner of its applicationto a carrier is such that a visually smooth coating or film is provided.However, it remains so that the coating will exhibit a microscopicdegree of roughness with peaks and troughs formed by carbon particles atthe exposed surface. It is then important to pay special attention tothe provision of an electrical connection to the coat or film.

According to yet another aspect of the invention an electricalconnection to a coat or film incorporating carbon particles is formed byfirst spraying a nickel compound to an area of the coat or film, andapplying to the sprayed area a tin-copper tape coated with a silverloaded conductive adhesive. With the electrical connection installed,the sheet or web can be connected to a suitable power source with thesubstantial guarantee that there will be no shorting or arcing at thepoint of electrical connection, and consequently no damage by thecreating of hot spots.

Dependant on the use to which the sheet or web is put, an electricalconnection can be provided over long lengths to opposite edges, toassist in the even input of power to the coat or film over a wide area,by locating a conductive rail on the silver loaded conductive tape.Desirably, the conductive rail is overlaid by an antifaying compound,preferably wider than the rail.

An important aspect of the invention is that the totally uniform heatingeffect achievable in the coat or film can be with relatively low power,eg 24 volts, and by controlling the width between the electrodes orrails, and the thickness of the coat or film, a constant temperature canbe achieved and maintained, at any required level to serve a particularpurpose.

In its form as a flexible sheet or web, an outer insulating layer can besprayed on to form a water/fluid resistant electrical insulator.Suitable materials may be polyurethane, silicone or acrylic elastomers.The invention has a considerable number of available uses. It can bewrapped round an article the temperature of which is to be maintainedand equally can be incorporated into clothing for use in extremely coldclimates, to maintain the temperature of the wearer.

However, the fabric has significant benefits when used in medicalcontexts. It can be incorporated in a mattress or as a blanket for anoperating table or for a bed in the recovery ward for raising andmaintaining the temperature of patients following surgery.

It is possible to employ the fabric in sleeping or carrying bags for useby rescue services to give immediate aid to accident victims sufferingfrom hyperthermia.

In all such uses, the low voltage required means that there is totalsafety to the user. The voltage and coat or film thickness can determinethe maximum and constant temperature across the full width and length ofthe fabric.

The arrangement may be that power is supplied to the electricallyconductive heater until such time as its required temperature is reachedand then maintained at that temperature by an appropriate switchingarrangement to switch power ON and OFF as required. It is howeverpreferred for power to be supplied as a series of pulses ofpredetermined time, with intervening periods where power is switched offfor predetermined periods of time, to allow temperature sensing to takeplace. Irrespective of the form of power supply, the invention allowshighly efficient use of electrical power.

In addition to being able to control maximum temperatures by voltagecontrol by control of the spacing between electrodes and by coatingthickness it is a highly advantageous feature of the invention that thepredetermined maximum temperature to suit the application is achieved inreasonable time from the onset of power, after which the temperature isregulated and maintained at its predetermined level. This can be ofconsiderable importance not only in medical contexts but also in such asthe food industry and particularly in food processing where a requiredtemperature must be achieved quickly and maintained.

In addition to its use in conjunction with a flexible fabric, the saidcompound can be used differently. For example, it may be sprayed ontoproducts whereby to provide heat uniformly over the whole surface of acomplex product. The said compound can be screen printed onto supportsurfaces, or can be directly coated on to a product surface. Equally itcan be calendered or hot melt coated from dry compound to produceflexible sheets, or can be applied by a powder coating technique toproduce heatable laminates.

A further possibility within the invention is to provide a said compoundwhere the polymer material into which the carbon black is stirred issuch as to make the compound suitable for moulding or casting. Thus,form-stable sheets or shapes of material can be produced that can beused in a number of industrial applications, such as sub floor heaterpads, sub soil heater pads, linings for fly ash hoppers in powerstations the temperature of which needs to remain constant to avoid theash from becoming damp, or placement around such as pump and valvecastings, to prevent them from freezing and hence malfunctioning. Inaddition the said compound can be moulded around vessels for warmingchemical or liquids.

One embodiment of the invention will now be described with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic part sectional perspective view of a mattress orblanket containing a conductive material in accordance with theinvention;

FIG. 2 is a schematic plan view of a mattress or blanket as in FIG. 1;

FIG. 3 is an upper and view of an electrical connection to theconductive material;

FIG. 4 is an exploded perspective view of the electrical connector ofFIG. 3;

FIG. 5 is an electrical block diagram illustrating an electrical circuitable to cause the heating of and the maintenance of a constanttemperature in the conductive material; and

FIGS. 6 and 7 are schematic illustrations of a blanket essentially asshown in FIG. 1 formed as a wrapping or a surround for a productstructure or pipe;

In FIG. 1, a blanket or mattress 1 has a core 2 formed by conductivematerial 3 on a fabric support 4. The conductive material was producedby the method hereinbefore defined, and applied to the fabric layer as aseries of coatings, with each coating heated to 110° C. to 150° C. bypassing through an oven or a series of ovens before the application of asucceeding coating. For medical use, the coating can be 144 micronsthick. The conductive material 3 on its fabric support 4 is overlaid byan electrical insulating layer 5, and both encased in a flame retardantinsulation 6, of greater thickness to the rear or non-operative side 7than to the front or operative side 8.

The whole composite is encased in an outer casing 9 of a polyurethanematerial, and the edges fully sealed around the full periphery of theblanket or mattress, to ensure that the blanket or mattress is totallywaterproof, and readily cleanable and sterilisable.

On the conductive material 3 and below the electrical insulating layer5, is a conductive rail 10 which, as is shown more particularly by FIG.2, lies along the edges of the conductive material along its two longerlengths. For medical use, the spacing between the rails may be 460 mms.

The provision of a guaranteed electrical connection to the conductiverail is of particular importance, and arcing or shorting of theelectrical supply must be avoided. Of a separate significance is thesupply of current to the conductive material within the blanket ormattress, and as shown in FIGS. 1, 3 and 4 this is achieved by applyinga coating 11 of nickel over the length and width of the conductivematerial to be occupied by the conductive rail 10 on to which is applieda silver loaded adhesive tape 12 to which the rail 10 is attached.Overlaying the rail 10 is an antifaying compound 13 to guarantee theabsence of any electrical arcing. Extending through co-operatingapertures in the conductive layer, the nickel coating and the adhesivetape is a conductive stud 14 extending to a stud base 15 soldered to therear face of the conductive material 3, the stud passing through thefabric backing to cap 16, the cap being crimped to provide a connectionto an electrical lead 17. FIG. 5 is an exploded view showing theconnection.

As indicated in FIG. 2, thermocouples or thermisters 18 chosen to suit aparticular use of the blanket or mattress (thermistors for sensitiveapplications and thermocouples for more robust applications), arestrategically located on the conductive material, with leads extendingto a connection 19. As indicated in FIG. 5, there is a control unit 20,from where emerge electrical leads 21 for the conductive studs 14, therebeing a one shot or resettable fuse 22 provided in the lead extending toone of the conductive studs 14.

The blanket or mattress discussed above is ideally suited to medical useto serve as a mattress or overlay for an operating table in a theatre ora recovery bed in a recovery room, or as a blanket to overlie a patientHere the more sensitive thermistor would be employed to sense thetemperature of the mattress overlay or blanket. As indicated in FIG. 5,the mattress overlay or blanket is connected to a control unit 23 inturn connected to a transformer unit 24 that itself is connected to amains supply. This ensures that low voltage supply, preferably at 24volts is supplied to the mattress overlay or blanket.

FIG. 5 illustrates in block diagram form the electrical circuit from themains to the blanket or mattress. At the transformer unit 24, there isthe mains input leading to a filter 25 and the low voltage transformer26. This ensures the feeding of 24 volt supply to the control unit 23.From the inlet, power is supplied to a switching circuit 27, and then toan over temperature isolation circuit 28 from where it progresses to apower output for connection to the mattress overlay or blanket as hasbeen described above. Simultaneously, power is supplied to a controlmodule 29 having a temperature control means 30 and outputs connected toa visual alarm 31 an audible alarm 32 and to the switching circuit 27.The temperature sensing thermistors (or thermocouples) 18 on themattress overlay or blanket are connected to the control unit 23, toprovide signals to the temperature isolation circuit 28 denotingtemperature. An alarm test 33 is provided on the unit, the alarm testbeing connected to the control module, and able to simulate an overtemperature condition.

Immediately prior to the onset of operations, the temperature controlmeans is set to a temperature required of the blanket or mattress,typically average human body temperature. The transformer unit and hencethe control unit and mattress overlay/blanket are connected to a mainssupply, and the alarm test activated to confirm that all circuits areactive.

The switching circuit is such that at the outset, power is supplied tothe mattress overlay or blanket for a predetermined period of time,typically one minute and switched off for a second predetermined periodof time, typically 10 seconds. During the period that power is switchedoff, the temperature sensed by the thermistors 18 is signalled via thecontrol module 29 to the over temperature isolation circuit 28, and ifthe temperature sensed is lower than the pre-set temperature, power isrepeatedly switched on and off and the temperature sensed, until thetemperature of the blanket or mattress is that required. When in thatcondition, power is held off until a fall in temperature of the blanketor mattress is sensed.

With a patient on the mattress overlay, or wrapped in the blanket, anyfall in body temperature causes an extraction of heat from the mattressor blanket, immediately sensed by the thermisters, and signals sent tothe over temperature control circuit and switching circuit to cause thepulsed supply of power to recommence until such time as the temperatureof the blanket or mattress recovers to the pre-set level.

By virtue of the method of producing the carbon laden material at itscarbon to polymer density, and as a consequence of the manner of itsapplication to a support fabric, there is the substantially uniformheating over the whole area of the blanket, with a substantially totalelimination of hot and cold spots the result of which is that there istotal support for the body heat of a patient over his or her full heightand width, ensuring that body temperature control crucial to a patientduring operations and subsequent recovery, is maintained.

In the most unlikely circumstances that the blanket or mattress shouldoverheat, both the visual and audible alarms are activated, and the overtemperature isolation circuit switched to prevent further power supplyto the blanket or mattress. To guard against over temperature beingconsequent on a momentary surge of power, over temperature sensed duringa first ten second switch-off of power can be caused to be ignored, andactivation of the alarms and temperature isolation circuit activatedonly if over temperature is sensed during the second of two successiveperiods of switch-off of power and temperature sensing.

The above described construction of blanket or mattress and its control,whilst ideally suited to medical applications, can be used withoutessential changes for other more industrial uses.

As is schematically suggested in FIGS. 6 and 7 a generally rectangularconstruction 34 of essentially the same nature as is illustrated in FIG.1 can be used to wrap round an item 35 that needs to be heated.

That item could be a domestic hot water tank, when a uniform heatingeffect is provided over its whole height, to the considerable benefit ofthe saving of power by creating a more rapid heat up from cold and a farmore efficient maintenance of the temperature of water in the tank.

The item could be e.g. pipework in, e.g. the food industry where theinvention can provide both an insulating lagging of pipework throughwhich heated and fluid foodstuffs must flow and the provision of uniformheating over the full pipe work length. Not only does this have majorimportance during normal operations, but should there be the need toclose down operations for any reasons, foodstuffs can solidify.Hitherto, solidified oil-based products in pipe lines has been a majorproblem. With the invention, recommencement of the provision of heat tothe blanket and hence to the pipework has the effect of gently andspeedily re-heating the foodstuff to bring it back to a fluid state whenflow can recommence.

The item could equally be one of outdoor use in extremely cold climatessuch as for example valves and pumps, that can be encased in thematerial of the invention to maintain them at a temperature that allowsthem to function normally no matter what the ambient temperature mightfall to.

What is claimed is:
 1. A flexible electrically beatable semi-conductivematerial, characterised by finely divided carbon particles uniformlydispersed in an elastomeric polymer there being carbon particle levelsof 20% by weight to 75% by dry weight to 80% to 25% by dry weight ofelastomeric polymer levels, the material also including ananti-adsorption compound selected from the group consisting ofpolypropylene glycols and polyethylene glycols.
 2. A conductive materialas in claim 1, characterised in that carbon particle levels of 20% to40% by dry weight to 80% to 60% by dry weight of elastomeric polymerlevels are used.
 3. A flexible conductive material as in claim 1,characterised in that it is in the form of conductive film or coatingand comprises a carbon filled elastomeric polymer with carbon particlelevels of 43% to 73% by dry weight to 57% to 27% by dry weightelastomeric polymer levels.
 4. A flexible conductive material as inclaim 1, characterised in that the carbon particle level is 57% by dryweight to 43% by dry weight of elastomeric polymer.
 5. A flexibleconductive material as claim 1, characterised in that the elastomericpolymer is an aliphatic polyurethane in solution.